University of Birmingham An experimental investigation of the aerodynamic flows created by lorries travelling in a long platoon Robertson, Francis; Bourriez, Frederick; He, Mingzhe; Soper, David; Baker, Christopher; Hemida, Hassan; Sterling, Mark DOI: 10.1016/j.jweia.2019.103966 License: Creative Commons: Attribution-NonCommercial-NoDerivs (CC BY-NC-ND) Document Version Peer reviewed version Citation for published version (Harvard): Robertson, F, Bourriez, F, He, M, Soper, D, Baker, C, Hemida, H & Sterling, M 2019, 'An experimental investigation of the aerodynamic flows created by lorries travelling in a long platoon', Journal of Wind Engineering and Industrial Aerodynamics, vol. 193, 103966. https://doi.org/10.1016/j.jweia.2019.103966 Link to publication on Research at Birmingham portal General rights Unless a licence is specified above, all rights (including copyright and moral rights) in this document are retained by the authors and/or the copyright holders. 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Take down policy While the University of Birmingham exercises care and attention in making items available there are rare occasions when an item has been uploaded in error or has been deemed to be commercially or otherwise sensitive. If you believe that this is the case for this document, please contact [email protected] providing details and we will remove access to the work immediately and investigate. Download date: 26. Sep. 2021 An experimental investigation of the aerodynamic flows created by lorries travelling in a long platoon Francis H. Robertson, Frederick Bourriez, Mingzhe He, David Soper, Chris Baker Hassan Hemida, Mark Sterling School of Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT Abstract The concept of autonomous road vehicles has recently gained a great deal of technical respectability. Expected advantages over normal driver-controlled vehicles are through increased safety, reliability and fuel efficiency. This paper presents a novel experimental study enabling for the first time a full understanding of the aerodynamic flow development of a long vehicle platoon. Moving model experiments were carried out at the University of Birmingham Transient Aerodynamic Investigation (TRAIN) rig facility on a 1/20th scale eight lorry platoon with three constant vehicle spacings. Slipstream velocity and pressures, as well as simultaneous on-board vehicle surface pressure measurements were made. Results indicated a highly turbulent boundary layer development, with slipstream pulse peaks near the front of each lorry; similar to previous findings on flows around container freight trains. The drag coefficient of an isolated lorry was in agreement with previous studies. There are substantial reductions in aerodynamic drag for the non-leading platoon vehicles. Drag results plateaued towards a constant value within the platoon. Vehicle spacing affected drag values, with decreases of 57% observed for the closest spacing of half a vehicle length, demonstrating the aerodynamic benefits of platooning. Keywords: Vehicle aerodynamics; lorry; platoon; slipstream; drag reduction; experimental study; model-scale. 1 Introduction In recent years the concept of driverless or autonomous road vehicles (AVs) has gained a great deal of technical respectability. Much progress has been made on a range of technologies relevant to this concept, including digital mapping, position recognition by lidar and radar systems and advanced vehicle-to-vehicle communications [1]. There are a number of advantages for such vehicles over normal driver-controlled vehicles in terms of safety, 1 reliability, access for the disabled and increasing the efficiency of road use [2]. The last of which comes about primarily because the vehicles are able to drive close together in platoon formation. For these reasons, a number of projects have trialled platooning technology. SARTRE (Safe Road Trains for the Environment) is a project by seven companies from four EU countries which aimed to develop a system that enabled platoons to operate on public motorways without any road infrastructure changes [3-5]. The lead vehicle is operated by a driver and the following vehicles are driven automatically by the system, following instructions from the lead vehicle [3]. This system has been successfully tested, with a five-vehicle platoon (2 leading trucks and 3 following cars), on test tracks and public motorways [3-5]. Safety analyses were also conducted to consider the effects of potential system faults, driver error and malicious third-party actions. Another large-scale EU project, COMPANION (Cooperative dynamic formation of platoons for safe and energy-optimized goods transportation), developed on- and off-board user interfaces and systems for the coordination of platoons of heavy-duty vehicles and validated the systems via simulations and trials on public roads [6]. The UK’s first heavy goods vehicle platooning trial, commissioned to show how AVs can improve safety and reduce emissions, is also underway [7]. TRL (Transport Research Laboratory) are overseeing the trials on both test tracks and major public roads for platoons of up to three partially self-driving vehicles, where drivers steer all the lorries but acceleration and braking are controlled wirelessly by the lead driver [8]. Platooning has also been explored in other countries, including Germany, Japan and the USA [8]. With projects such as these either completed or underway, it is clear that the use of AVs will soon be widespread. Indeed, it has been estimated that AVs will become commercially available by 2025 [9], with potential economic benefits reaching $200 billion to $1.9 trillion per year [10]. The concept of running vehicles in close formation or a platoon is not an entirely new phenomena wholly associated with AVs. Indeed, it is commonplace to see lorries travelling in tandem on motorways and the concept of using the draft/slipstream for overtaking (one vehicle following closely behind another and travelling in its slipstream to achieve aerodynamic benefits) is commonplace in sports car racing [11] and cycling [12]. When this occurs, there is an increase the pressure at the rear of the leading vehicle due to flow stagnation on the vehicle behind and a reduction in the momentum of the flow approaching the vehicle behind [5,11]. This tends to reduce the drag on both vehicles, which may improve fuel efficiency with benefits such as cost reductions and reduced carbon dioxide emissions. To this end, a number of studies have looked at fuel or power consumption in full-scale vehicle platoons [3-5,13-14]. Indeed, SATRE showed that greater drag reductions and fuel savings can be achieved with smaller inter-vehicle spacing and estimated that by platooning, trucks could save up to 2.8 tons of CO2 equivalent per year and cars up to 0.1 tons [3,5]. Furthermore, track tests by Veldhuizen et al. [14] showed that there were significant fuel savings (up to 11.7 ± 0.9%) for the trailing vehicle in a platoon of two heavy duty vehicles. However, for this short platoon formation, there were no significant gains as the vehicle spacing was reduced from the current European legal limit of 50 m down to 10 m. For the leading vehicle there were significant gains at close spacing, but the fuel savings were small in comparison to those of the trailing vehicle. To date only a restricted amount of experimental work has been carried out on the aerodynamics of vehicles travelling in platoons and as such the nature of the flow field 2 development around a platoon is not well understood. The PATH project [15] investigated various aspects of the aerodynamic effects due to platooning using wind tunnel tests on 1/8th Lumina APV models. The number of vehicles in the platoon varied from 2 up to 4, while the inter-vehicle spacings tested were from 0 up to 3 times vehicle length. The results suggested that significant drag reduction could be achieved, especially in the strong interaction regime case where the inter-vehicle spacing is less than one vehicle length, and more complicated drag behaviour was identified for the cases with inter-vehicle spacing less than half vehicle length. The drag benefits of platoons with different vehicle shapes were also confirmed by other wind tunnel studies [16-18], although the research focused on different aspects such as heterogeneity, and in-line oscillation. Benefits were observed for 1997 Buick LeSabre scale car models, rectangular box models with sharp corners and edges (designed to simulate mini-vans or a buses) [16] and Ahmed bodies [17-18]. It should be noted however, that studies have shown that platooning does not always lead to drag reduction. Experimental results for a platoon of two Ahmed-bodies with a slant angle back [17-18] indicated a drag penalty for the trailing vehicle at small inter-vehicle spacing, opposite to the findings of Zabat et al. [15] for a square back two-vehicle platoon. For backlight angles and vehicle spacings between 25° to 35° and 0.125 to 3 vehicle lengths respectively, no combination yielded drag reductions for both models [17]. Overall benefits were observed for spacings less than half the vehicle length, but this included a significant increase in drag for the trailing vehicle (6-42%).